Attenuation for crossed-field devices



June 21, 1966 P. w. CRAPUCHETTES ATTENUATION FOR CROSSED-FIELD DEVICES Filed Dec. 18. 1961 United States Patent 3,257,575 ATTENUATIGN FOR CRflSSED-FIELD DEVICES Paul W. Crapuehettes, Atherton, Califi, assignor to Litton Electron Tube Corporation, San Carlos, Calif. Fiied Dec. 18, 1961, Ser. No. 160,025 6 Claims. (8!. 315-35) This invention relates to attentuation for traveling-wave devices and more particularly to an attenuation arrangement for crossed-field type travelling-wave devices to absorb undesirable electromagnetic energy propagating in the slow-wave circuit of the device.

Travelling-wave type electron discharge devices or tubes, in general, have had a slow-Wave transmission circuit, such as a helical structure or an interdigital structure, and an electron beam in interacting relationship with an electromagnetic wave propagating along the slow-wave transmission circuit and have been employed to produce amplification of a microwave frequency signal or the generation of oscillations in such devices. In general, travellingwave type devices have their elements enclosed in an evacuated envelope which may include an electron gun at one end of the slow-wave structure, an anode at the other end of the slow-Wave structure for collecting electrons from the electron gun, and an arrangement coextensive and adjacent a portion of the slow-wave circuit for attentuating undesirable electromagnetic energy propagating in the slow-wave circuit and for terminating a slow-wave circuit when it is utilized as a backward wave oscillator.

Whenever such tubes are employed as a crossed-field backward wave oscillator, the tube includes a cylindrical periodical anode slow-wave transmission line circuit surrounding and concentrically arranged with respect to a negative electrode or sole electrode thus forming an arcuate interaction space. A heated cathode positioned adjacent one end of the interaction space serves as a source of electrons. These electrons, under the combined influence of an electric field established between the slowwave circuit and the sole electrode which is negative with respect to the slow-wave circuit, and a magnetic field transverse to the electric field, are permitted to encircle the arcuate interaction space in energy coupling relationship with an electromagnetic wave propagating in the slowwave transmission circuit, providing that the magnitude and polarity of the two mutually perpendicular fields are properly adjusted.

In order for these crossed-field travelling-wave devices to be of practical value, they must be electrically stable and free from spurious oscillations in operation. In accordance with the prior art, spurious oscillations propagating in the slow-wave circuit have been eliminated by introducing a circuit-loss-medium along the slow-wave structure of the device to absorb or attentuate undesirable energy propagating in a preselected direction. In the instance of the interdigital slow-wave structure, which has generally been utilized for crossed-field backward wave oscillator devices, a preselected portion of the structure is coated with a lossy material or a portion of the interdigital elements are made of a lossy material, such as iron for example. Both of these techniques require the attenuation material to extend a relatively long distance along the length of the slow-wave structure in order to provide the degree of impedance matching or attenuation desired. In general, at low power levels the problem of providing adequate attenuation within the vacuum envelope adjacent the interaction space is not a serious one. However, at relatively high power levels, the attenuation of spurious oscillations and the dissipation of heat resulting from such attenuation causes several serious problems. One of these problems arises from the fact that the higher power devices, such as 400 watts continuous wave (C.W.) power at 3,257,575 Patented June 21, 1966 500 milliamperes current for example, requires either an unusually long length for the attenuation arrangement or a material which has a very high attenuation capability and therefore will attenuate a great amount of power in the length along the slow-wave structure allocated for attenuation. Another problem arises from the fact that the prior art attenuation arrangement of the interdigital slow-wave structure, for example, is within the vacuum envelope adjacent tht interaction space, and, therefore, is not readily adaptable to arrangements for cooling which may be outside of the vacuum envelope or remote from the interaction space. I

In practice, it has been found that these problems are dependent upon one another. In particular, if the attenuation arrangement includes a material disposed along the slow-wave structure adjacent the interaction space which is capable of providing the required attenuation, several disadvantages may result. First, the amount of heat generated in the attenuation arrangement may be greater than the cooling arrangement is capable of dissipating, which in turn would cause the over-heating of the slowwave circuit of the tube. In the instance where the attenuation arrangement comprises iron plating formed on a preselected number of the interdigital elements or fingers at the end of the slow-wave structure near the collector anode, these elements may become distorted or deranged to such an extent that they would project into the path of the electrom beam which would melt them. This melting of the structure by the electrons may cause the interdigital finger to short-out portions of the circuit since they are closely spaced, and cause undesirable impedance mismatching. This advantage may be reduced or minimized by increasing the total number of elements utilized in the attentuation arrangement; However, this would create other disadvantages; that of reducing the number of interdigital elements along which the electron beam interacts with the travelling-wave on the structure, or increasing the overall length of the slow-wave circuit. A reduction in the active length of the slow-wave structure reduces the length of the interaction space which would in turn reduce the power generating capabilities of the device, while increasing the overall length and the size of the tube.

The reduction in power generating capacity may be overcome by increasing the length of the slow-wave structure to provide a greater length of the interaction space along which the electron beam has for interacting with the wave travelling on the structure. However, an increase in the length of the slow-wave structure is objectionable because it would be extremely difficult, if not impossible, to provide a very tightly bunched electron beam which would traverse the full length of the interaction space without spreading and impinging upon the structure along its path. Furthermore, the increased length in the slow-wave structure is objectionable because it would significantly increase the weight of the travelling-wave device. This increase in weight will be especially objectionable in airborne applications in which size and weight are of primary importance. From the foregoing discussion it becomes apparent that the prior art attenuation arrangements have several severe disadvantages when considered for use in applications demanding high power capabilities, good dissipation capabilities, light weight and small size, such as air-borne applications for example.

The present invention obviates the foregoing and other disadvantages of the prior art by providing an attenuation arrangement which is discretely coupled to the slow-Wave circuit, and which has an extended lossy region for dissipating energy at a place remote from the slow-wave circuit. In accordance with an illustrative embodiment of the present invention, there is provided an attenuation arrangement which includes a lumped lossy dielectric material disposed at the end of the slow-wave structure remote from the cathode end of the tube, and a coaxial attenuator having a toroidal configuration comprising a long slender inner electrical conductor, an outer electrical conductor substantially surrounding the inner conductor, and a lossy dielectric material disposed between the inner and outer conductors completely surrounding the inner conductor.

More particularly, the toroidal attenuation arrangement is disposed in concentric spaced relationship with the slowwave structure and interaction space of the device and remote therefrom. The inner coaxial element may be a slender electrically conductive wire which is preferably more than one wave length long measured with reference to the center of the operating frequency of the device, and is connected at one end of one of the last interdigital fingetrs of the slow-wave circuit, while the other end is attached by a ground connection to the body of the tube. The outer element of the attenuation arrangement, which has a channel-shaped configuration, encompasses the inner conductor and is affixed to one of the crowns of the device on the side away from the interaction space. The crowns for a crossed-field backward wave oscillator are generally parallel plates for supporting .the interdigital fingers of the slow-wave structure, and are disposed axially to the tube, one on each side of the interaction space and concentric therewith. When the interdigital elements are connected to the crown, the crown becomes a part of the slow-wave structure, and the attenuation arrangement is affixed concentrically to the slow-wave structure on the side away from the interdigital elements. The inner wire conductor is separated from the outer channel element by a lossy dielectric material, such as powdered nickel and ceramics, in which a substantial portion of the undesirable oscillation energy is absorbed.

Since the attenuation arrangement is remote from the interaction space, it may be more accessible and therefore may be cooled more readily by any suitable cooling arrangement which may be placed in thermal conductive cont-act with the exposed surface of the attenuation arrangement. For example, the toroidal attenuation arrangement may be surrounded by a liquid cooling jacket in contact therewith to remove the heat generated therein by energy absorbed in the lossy material of the arrangement.

It is, therefore, an object of this invention to provide an improved attenuation arrangement for high power travelling-wave devices to prevent undesirable oscillation in the slow-wave transmission circuit.

Another object of the invention is to provide an improved attenuation arrangement in the form of a coaxial line to substantially extend the energy absorption capabilities of crossed-field travelling-wave devices while limiting the weight and size of such travelling-wave device to a minimum.

A further object of the invention is to provide an improved attenuation arrangement which is remote from the interaction space of a travelling-wave device thereby enabling heat generated within the attenuation arrangement to be removed more readily and efficiently than with prior art arrangements.

Still another object of this invention is to provide an improved attenuation arrangement for high power crossedfield travelling-wave tubes in which absorption of undesirable energy is accomplished without loss of power generating capabilities and efiiciency over a wide frequency band. F

The novel features which are believed to be characteristic of the invention bOllh as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly 4 understood, however, that the drawings are for purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

FIGURE 1 is a diagrammatic view, partly in crosssection, of a crossed-field backward travelling-wave oscillator tube illustrating the manner in which a toroidal coaxial attenuation arrangement is utilized in accordance with the invention;

FIGURE 2 is a fragmentary diagrammatic view, partly in cross-section, of the attenuation arrangement and the slow-wave structure shown in FIGURE 1', and

FIGURE 3 is another fragmentary diagrammatic view, partly in cross-section, of the attenuation arrangement and the slow-wave structure shown in FIGURES 1 and 2.

Referring now to the drawings wherein like or corresponding parts are designated by the same reference characters throughout the several views, there is shown in FIGURE 1 a crossed-field backward wave oscillator device 10 whioh is operable to generate a microwave oscillation signal. As shown in FIGURE 1, device 10'includes an evacuated envelope, having a side wall 12 and a pair of enclosure ends 13 of which only one is shown, for enclosing an electron gun, the input of which is shown as input socket 14, including a cathode and control electrode for producing and forming an electron beam, a slow-wave transmission circuit 16, commonly called an interd-igital structure which extends substantially concentrically with a sole electrode 18 which cooperates with the slow-wave circuit to form an interaction space and produce an electric field between the sole and slow-wave circuit. The device further includes a coaxial output coupler 20 for coupling microwave energy generated within the device to an external load, an attenuation arrangement 22, an electrode 23 for collecting the electron beam, a steady biasing magnetic field illustrated by one pole piece 24 of a permanent magnet or electromagnet and a pair of parallel crown members 26 disposed concentric to the device and perpendicular to the axis of the tube. The slow-wave structure 16 may be formed by afiixing a plurality of interdigital elements 27 alternately to the upper and lower support crown elements, axially, and concentrically arranged with respect to the sole 18. The magnetic source includes two opposing pole pieces, only one of which is shown for convenience in i order to simplify the drawings, providing magnetic flux lines within the electron gun region and the interaction space of the device normal to the plane of the circular dimension of the tube.

It should be noted at this point, that the oscillator shown in the accompanying drawings is in the form of a circular device. However, these devices may be constructed in a linear configuration. When a linear configuration is employed, the magnetic and electric fields are adjusted in accordance with known electron optic principles to provide suitable trajectory for the electron beam.

Turning now to the details of the elements of the attenuation arrangement 22 shown in FIGURE 1, as shown drawn to a larger scale in FIGURES 2 and 3, it can be seen that the attenuation arrangement includes a lumped attenuation member 28 at the end of the slowwave circuit 16 adjacent anode electrode 23. The member 28 aids in matching the impedance of the coaxial attenuator 22 thereby eliminating significant electromagnetic energy wave reflections in the device while also providing a significant amount of attenuation to undesired spurious oscillations. In this illustrative embodiment, the remainder of the attenuation includes an outer channel or U-shaped element 30 which substantially surrounds an inner wire conductor 32 which is embedded in a lossy dielectric material 34 and separates the two conductors thereby forming a coaxial line arrangement. As shown in FIGURE 1, the attenuator designated 22 has a toroidal configuration and is afiixed to one of the crowns 26 on the side opposite the i-nterdigital fingers 27. The length of the coaxial attenuator shown in FIGURE 1 is about two wave lengths long measured with reference to the center of the operating frequency of the tube. This length is not to be construed as a Limitation since the length may be increased whenever greater attenuation is required, provided, that the configuration is modified to provide the added length.

Continuing with the description of the invention with reference to FIGURE 2, one end of the inner wire conduotor 32 is connected to the end of one of the interdigital elements 27. The point of connect-ion between the wire 32 and a finger designated 36 is designated 38. One quarter of a Wave length from connection 38 along the zigzag path of the circuit is a surface designated 40 on the surface of crown 26 which terminates the slow-wave circuit. The one quarter wave length is measured with reference to the center of the operating frequency band of the tube. The one quarter wave length distance between connection 38 and surface 40 is selected to provide a choke arrangement for matching purposes to eliminate undesirable reflections between the connection 33 and surface 40. In the drawing, the other end of the inner conductor designated 32 is schematically shown as a grounded connection. However, in practice the connection is made to the body of the tube at any convenient location.

It should be noted at this point, that although one end of the inner conductor is connected to the slow-wave circuit at a point about one quarter of a wave length from the termination end of the slow-wave line, it may be attached at another discrete point. For example, if the attenuation arrangement is employed in an amplifier device, the connection would be made preferably near the middle of the length of the slow-wave line. Such a connection would be in accordance with the practice of inserting the attenuation at about the midpoint of the interaction space to provide enough length of slow-wave structure beyond the insertion of the attenuation to insure the desired amplification of the input signal.

As shown in FIGURES 2 and 3, the inner and outer conductors 32 and 30 respectively, are separated by any suitable lossy dielectric material 34. For this illustrative embodiment, a suitable alumina ceramic having by weight the following elements has given satisfactory performance:

Pencent Ceramic material 45 Consisting, by weight, of about:

Alundum, 38-900 acid treated percent 97.90

Yellowstone Talc do 2.06

Kentucky Ball Clay, No. 4 do 0.04

100 Pure nickel powder 45 Zirconium oxide The Alundum, 38-900 acid treated, is available [from the Norton Co. of Worcester, Massachusetts; the Yellowstone Tale is available from the Sierra Talc and Clay Co. of 1608 Huntington Drive, South Pasadena, California; and the Kentucky Ball Clay, No. 4, is available from the Western Ceramic Supply Co. of 1601 Howard Street, San Francisco, California.

It should be noted at this point that the energy absorption capabilities of the attenuator arrangement may be increased by significantly increasing the over-all length of the coaxial attenuator or the length of the inner conductor of the arrangement. One method for accomplishing this end is that of utilizing a slender helical coil in place of the single inner wire conductor. Another method of increasing the absorption capabilities of the arrangement is that of plating the inner conductor, which may be a high resist-ant tungsten Wire about 0.010 inch in diameter, or the inner sunface of the outer conductor or both with a lossy material such as iron for example. Both of these methods will significantly increase the resistance of the arrangement when utilized, and thereby increase the energy absorption properties of the arrangement. The amount of lossy material plated onto the conductors or the added length of the inner conductor or both will depend upon the amount of absorption desired. 7

With reference to FIGURE 3, the description of the attenuation arrangement will be continued. As shown in FIGURE 3, the lumped attenuation member 28 has a preselected tapered configuration which functions as an impedance matching device between the slow-wave circuit and the coaxial attenuator. The lumped attenuation, which is employed also as a lossy material for attenuating spurious oscillations, may be of the same or similar materials as that utilized for the dielectric material used to separate the inner and outer conductors of the coaxial attenuator set forth hereinab-ove. The lumped attenuation 28 has several rectangular slots cut therein parallel to the axis of the tube to accommodate the interdigital elements at the termination end of the slow-wave circuit. The lumped member 28 is aflixed to the collector electrode 23 at its remote end, and to said side wall 12 of the tube. Also shown in the drawings is the collector 23 which has a tapered surface designated 42 upon which the electrons from the electron beam impinge and are collected after they have traversed the interaction space. Consider now the electrical operation of a crossed-field backward Wave oscillator device in which there is provided an attenuation arrangement as taught by the present invention. Operation of the tube is based upon the interaction which will occur between an electron beam and the wave energy propagating in the slow-wave circuit in the backward d-irectioi. It is expressly understood that the invention is not limited in its application to crossed-field backward travelling-Wave oscillators, but it is also applicable to other travelling-wave type tubes.

In operation, the oath-ode is activated to provide a stream of electrons which are injected into the interaction space between the slow-wave circuit 16 and the sole electrode 18 where they interact with the backward travelling electromagnetic wave energy induced and propagating in circuit 15. As the oscillations of the tube build up and energy is coupled to an external load, reflected energy waves occur at the output which travel back down the circuit in the forward direction toward the remote end of the circuit. These reflected energy waves are attenuated by the lumped and coaxial attenuators of the attenuation ar'nangement. The arrangement provides a minimum of 30 db attenuation for the reflected forward wave in the iforward direct-ion and a similar amount in the reverse direction.

Consider now advantages which are derived through the use of the novel form of attenuation arrangement in accordance with the teachings of the present invention.

I From a mechanical point of view, the coaxial attenuator has the advantage of providing an arrangement which is simple and easy to construct providing apparatus for significantly reducing the size and weight of the tube. In particular, it has been calculated for high power operation, such as 400 Watts (C.W.), that it would be necessary to at least double the length of the slow-wave circuit, at L-band for example, to provide a structure long enough to accommodate the required attenuation arrangement adjacent the interaction space. Such an increase in the length of the slow-wave structure would make the size and weight of the device prohibitive for ainborne applications where size and weight are of paramount importance.

Another advantage of the present invention arises from the fact that a substantial part of the attenuation arrangement is remote from the interaction space and is, therefore, accessible to cooling apparatus which may be designed to readily remove the heat therefrom.

Still another important advantage of the present invention is that of providing a method by which the amount of attenuation added to the device may be controlled very accurately. More particularly, the impedance characteristics of the coaxial attenuator of the invention may be determined rather precisely, by variations in the lossy dielectric material, the plating on the inner or'outer conductor or the length of the inner conductor when a slender helical coil is utilized as the inner conductor. In addition, it is possible to change the attenuation arrangement should the necessity arise without the necessity of changing a slow-wave structure which may have already been incorporated into a tube. This advantage means that any discrepancies in impedances which may arise between the slow-wave circuit from tube to tube due to manufacturing processes may be compensated for by modifying the impedance of the coaxial attenuation to make the two elements as compatible as possible.

While the attenuation arrangement of the invention has been described with reference to a coaxial attenuator affixed to one of the crowns of a crossed-field backward wave oscillator, it is to be understood, of course, that further alterations and modifications may be made in the attenuation arrangement shown and that they also may be utilized in any type of travelling-wave device where attenuation of undesirable oscillations due to reflected energy waves occur. Thus, by way of example but not of limitation, the coaxial attenuation arrangement may be modified to fit around the periphery of the vacuum envelope in a plane perpendicular to the axis of the tube. Such an arrangement would permit the addition of as much attenuation as required while maintaining a minimum size and weight as possible. Accordingly, it is to be expressly understood that the invention is to be limited only by the spirit and scope of the appended claims.

What is claimed as new is:

1. In a travelling-wave electron discharge device having an energy absorbing means for impedance matching of a slow-wave transmission circuit, said slow-wave circuit including:

a pair of parallel crown elements, and

a plurality of interdigital elements afiixed to said crown elements in alternate spaced relationship forming thereby a zig-zag path for propagating an electromagnetic wave and for introducing a delay in the propagation of said wave, said absorbing means comprising:

a lumped lossy dielectric material disposed at the remote end of the slow-wave circuit, and

a coaxial attenuator including a long slender metallic conductor and an outer metallic conductor substantially surrounding the inner conductor and a lossy dielectric material disposed between the inner and outer conductors completely surrounding said inner conductor, said coaxial attenuator being affixed to one of said crowns on the side opposite said interdigital elements in concentric spaced relationship with said slow-wave circuit.

2. In combination:

a slow-wave transmission line circuit in the form of an interdigital structure,

a sole electrode in spaced relationship with said slowwave circuit forming an interaction space therebetween,

a cathode for producing and directing an electron beam along said slow-wave circuit through said interaction space to an anode for collecting said electrons, and

attenuation means for terminating said slow-wave circuit with a substantially matching impedance and for absorbing undesirable spurious oscillations,

said interdigital structure being formed by a pair of parallel elements having a plurality of interdigital elements alternately connected to said parallel elements forming a zig-zag path along the interaction space to enable said electron beam to interact with an electromagnetic wave propagating along said zigzag path, said attenuation means comprising:

a lossy dielectric material disposed at one end of said slow-wave structure adjacent said anode electrode, and

a coaxial attenuator having an inner electric conductor at least one wave length of a predetermined frequency long and an outer electrical conductor surrounding said inner conductor, and a lossy dielectric material disposed between said inner and outer conductors,

said coaxial attenuator being connected to one of said parallel elements on the side opposite said interdigital elements in concentric spaced relationship with said interdigital structure and interaction space.

3. In combination:

a slow-wave transmission interdigital structure, a sole electrode in spaced relationship with said slowwave circuit forming an interaction space therebetween,

a cathode for producing and directing an electron beam along said slow-wave circuit and through said interaction space to an anode for collecting said electrons, and

an attenuation means for terminating said slow-wave circuit with a substantially matching impedance and for absorbing undesirable spurious oscillations,

said interdigital structure being formed by a pair of parallel elements having a plurality of interdigital elements alternately connected to said parallel elements forming a zig-zag path along the interaction space to enable said electron beam to interact with an electromagnetic wave propogating along said zigzag path, said attenuation means comprising:

a lossy dielectric material disposed at one end of said slow-wave structure adjacent said anode electrode, and

a coaxial attenuator having an inner electrical conductor at least one wave length of a predetermined frequency long, an outer electrical conductor surrounding said inner conductor, and a lossy dielectric material disposed between said inner and outer conductors,

said coaxial attenuator being disposed remote from said slow-wave circuit in concentric spaced relationship with said slow-wave circuit and interaction space,

said inner conductor being connected to one of said interdigital elements at a point one quarter of a wave length of said predetermined frequency from the termination end of said slowwave circuit adjacent said anode electrode, said quarter wave distance measured along said zigzag path forming a radio frequency choke to eliminate reflections of electromagnetic energy waves between said coaxial attenuator and the termination end of said slow-wave structure.

4. A crossed-field backward wave oscillator tube having a vacuum envelope including:

. a slow-wave transmission line circuit in the form of an interdigital structure,

a sole electrode in spaced relationship with said slowwave circuit forming an interaction space therebetween.

means for producing and directing an electron beam along said slow-wave structure through said interaction space to an anode for collecting said electrons, and

line circuit in the form of an attenuation means for terminating said slow-Wave circuit with a substantially matching impedance and for absorbing undesirable spurious oscillation energy.

said interdigital structure being formed by a pair of parallel elements having l2], plurality of interdigital elements alternately connected to said parallel elements forming a Zig-zag path along the interaction space to enable said electron beam tointeract with an electromagnetic wave propagating along said zigzag path, said attenuation means comprising:

a lumped lossy dielectric material disposed at one end of said slow-wave structure adjacent said anode electrode, and

a coaxial attenuator having a toroidal configuration including a slender inner metallic conductor at least one Wave length of the central operating frequency of said tube long and an outer metallic conductor surrounding said inner conductor, and a lossy dielectric material disposed between said inner and outer conductors,

said coaxial attenuator being aflixed to one of said parallel elements on the side opposite said interdigital elements in concentric spaced rela-- tionship with said interdigital structure and in teraction space, said inner conductor being connected to one of said interdigital elements at a point one quarter of a wave length of said central operating frequency from the end of said slow-Wave circuit adjacent said anode electrode, said quater Wave distance measured along said zigzag path forming a radio frequency choke to eliminate reflections between said coaxial iattenuator and the end of said slow-wave structure. 5. A crossed-field backward wave oscillator tube including:

. a slow-Wave transmission line circuit,

means for producing and directing an electron beam :along said slow-Wave transmission line circuit toward an anode electrode for collecting said electron, and an attenuation arrangement, said arrangement comprisa lumped lossy dielectric material disposed at one end of said slow-wave circuit, and

a coaxial attenuator including an inner conductor and an outer conductor, and a lossy dielectric material separating said inner and outer conductor, said inner conductor being connected to said slow-wave circuit at a point one quarter of a Wave length of the central operating frequency of said tube from the termination of said circuit,

thereby forming a quarter wave length radio frequency choke to eliminate electromagnetic Wave energy reflections between said inner conductor and the end of said slow-Wave transmission line circuit adjacent said anode electrode.

6. In traveling wave electron discharge device having a slow wave transmission circuit,

means for attenuating the propagation of undesired electromagnetic Waves on said slow wave transmission line circuit, said means comprising:

a coxial transmission line having an inner and outer conductor and having the space between said inner and outer conductors filled with a lossy dielectric material, and

means for attaching said coaxial transmission line to a predetermined point on said slow wave transmission circuit.

References Cited by the Examiner UNITED STATES PATENTS HERMAN KARL SAALBACH, Primary Examiner. JAMES KALLAM, Examiner.

S. CHATMON, JR., C. O. GARDNER,

Assistant Examiners. 

1. IN A TRAVELLING-WAVE ELECTRON DISCHARGE DEVICE HAVING AN ENERGY ABSORBING MEANS FOR IMPEDANCE MATCHING OF A SLOW-WAVE TRANSMISSION CIRCUIT, SAID SLOW-WAVE CIRCUIT INCLUDING: A PAIR OF PARALLEL CROWN ELEMENTS, AND A PLURALITY OF INTERDIGITAL ELEMENTS AFFIXED TO SAID CROWN ELEMENTS IN ALTERNATE SPACED RELATIONSHIP FORMING THEREBY A ZIG-ZAG PATH FOR PROPAGATING AN ELECTROMAGNETIC WAVE AND FOR INTRODUCING A DELAY IN THE PROPAGATION OF SAID WAVE, SAID ABSORBING MEANS COMPRISING: A LUMPED LOSSY DIELECTRIC MATERIAL DISPOSED AT THE REMOTE END OF THE SLOW-WAVE CIRCUIT, AND A COAXIAL ATTENUATOR INCLUDING A LONG SLENDER METALLIC CONDUCTOR AND AN OUTER METALLIC CONDUCTOR SUBSTANTIALLY SURROUNDING THE INNER CONDUCTOR AND A LOSSY DIELECTRIC MATERIAL DISPOSED BETWEEN THE INNER AND OUTER CONDUCTORS COMPLETELY SURROUNDING SAID INNER CONDUCTOR, SAID 